Portacio was born in Sangley Point, Cavite City, Philippines. He grew up in places such as Midway Island and New Mexico before his family settled in San Diego, California. He began reading comic books at the age of ten, when his neighbor decided to get rid of her husband’s collection, and through this, he discovered the work of creators such as Jack
Zeck began his comics career in 1974, doing illustration assignments for the text stories in Charlton Comics’ animated line of comics, which led to work on their horror titles. During this period he lived briefly in the Derby, Connecticut, area where Charlton was headquartered, rooming with writer-artist-animator Daniel Reed.
In 1977, Zeck started working for Marvel Comics on Master of Kung Fu with writer Doug Moench. In 2010, Comics Bulletin ranked Moench and Zeck’s work on Master of Kung-Fu sixth on its list of the “Top 10 1970s Marvels”. Zeck later worked on Captain America and drew covers for G.I. Joe: A Real American Hero.
Zeck illustrated the Marvel Super-Heroes Secret Wars limited series in 1984. For this series, he and fellow artist Rick Leonardi designed a new black-and-white costume temporarily worn by Spider-Man. The costume began as a design by Zeck that Leonardi embellished. The plot that developed as a result of Spider-Man’s acquisition of the costume led to the creation of the Spider-Man villain known as Venom although in a 2007 Comic Book Resources story, fan Randy Schueller claims to have devised a version of a black costume for Spider-Man in a story idea that he was paid for.
In 1986, Zeck collaborated with writer Steven Grant on a Punisher miniseries which was later collected as The Punisher: Circle Of Blood and an original hardcover graphic novel of the character three years later.
Zeck illustrated the 1987 Spider-Man storyline “Kraven’s Last Hunt”, written by his former Captain America collaborator J. M. DeMatteis, which is considered to be one of the quintessential stories in Spider-Man’s history, as well as the definitive Kraven the Hunter storyline. DeMatteis remarked, “Because Mike nailed the plot elements so perfectly in his pencils—every action, every emotion, was there, clear as a bell—I didn’t have to worry about belaboring those elements in the captions or dialogue. I was free to do those interior monologues that were so important to the story. If any other artist had drawn “Kraven’s Last Hunt” … it wouldn’t have been the same story.” In 2004, Zeck’s cover of Web of Spider-Man #32, which depicts Spider-Man escaping the grave into which he has been interred by Kraven, was recreated as a 12-inch tall resin diorama statue by Dynamic Forces.
Zeck has worked for DC Comics as well. He contributed to Who’s Who: The Definitive Directory of the DC Universe in the mid-1980s. Zeck drew the covers for the “Ten Nights of the Beast” storyline in Batman #417-420 (March-June 1988) and these covers were later collected in a portfolio. His other credits for the publisher include Batman: Legends of the Dark Knight, Legends of the DC Universe, and Deathstroke, The Terminator. In 1999, he collaborated with writer Mark Waid on The Kingdom, a sequel to Kingdom Come.